Quite The Collection

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Purchase all available weapons.

There are 23 weapons you have to purchase and all together they cost 210,000 XP.

You can average between 45-50,000+ XP per campaign run on normal and even more on higher difficulty levels. You can also earn XP in Multiplayer Public and Multiplayer Private matches. This might take a while, but it's not hard, just time consuming.

The best place I have found to XP farm by far is on End of the line. Set your difficulty to casual and rock up a riot gun or juggernaut, the proceed to the to the part where you go to the main train-yard with Leon sniping at you and the special forces are boxed in. After where all the mines are go to the infinite ammo carte and make Leon appear, then drop back so Leon can't shoot you and get ready for infinite of crimson heads to come running round the corner in to your shot gun :D. The best person I found to use was Bertha because she can hold up to 4 first aid sprays and add 50% extra to herbs and sprays. You can roughly get 600 kills in 20-25 mins which can get up to about 20K per half hour.

easy way to sort of "cheat" these buying achievements. go through the entire game without really spending any money. by the last stage you should have roughly 50000 XP if completed on normal. go back into free play once completed the game, then buy all abilities. once the achievement pops up, go back to the dash board and restart the game. go back into free play and your 50000 XP will still be there. then upgrade all abilities for one single player. achievement pops up, back to dashboard again. restart then do the same thing with buying 5 guns and buying 15 guns achievement. need roughly 2 play thoughs for all guns and upgrade all abilities for all players achievemt.